The Orthogonal Frequency Division Multiplexing (OFDM) modulation is a widely developing technique in the field of wireless digital communications thanks to the high possibilities offered by simplified digital signal processing based on Discrete Fourier Transform computations.
However, before a signal being received by a User Equipment (UE) may be properly processed, a preliminary but critical synchronization step has to be achieved including the estimation of the so-called Carrier Frequency Offset (CFO). Indeed, because of the mismatches of the internal oscillators and also because of the Doppler effect resulting from mobile communications, the frequencies at the base station (eNodeB) and at the receiving terminal (UE) may differ, and such a difference might be detrimental to the targeted transmission rate. In particular, if the CFO problem is not properly corrected, it is hazardous to perform the Discrete Fourier Transform stage at the receiver. In many systems, such as 3GPP-LTE, a first rough synchronization step is performed thanks to a primary synchronization sequence (PSS) which at this point is the only known pilot sequence accessible to the receiver.
Before performing the next synchronization steps which require to use the DFT block at the reception, it is critical to have a good CFO estimate; otherwise the performance of the subsequent synchronization steps might be heavily impaired.
A first, very well known, technique for estimating the CFO is disclosed by P. H. Moose in “A technique for orthogonal frequency-division multiplexing frequency offset correction”, in IEEE Trans. on Communications, vol. 42, no. 10, pp. 2908-2914, October 1994. The technique is based on the use of a specific pilot sequence, a pilot OFDM symbol composed of two identical vectors.
Such a technique presents the significant advantage of being workable without the knowledge of the channel characteristics and, therefore, is highly useful during the first or coarse synchronization process when only limited information is available to the receiver. The clear drawback of this well-known technique lies in the fact that the repetitive transmission of the Moose sequence is a large waste in terms of spectral efficiency since this sequence is not in general reusable to other synchronization purposes.
For such a reason in particular, the use of the Moose sequence has been excluded in some standards, such as the 3GPP-LTE for instance, and there is only provided the so-called Primary Synchronization Sequence (PSS) for achieving the estimation of the CFO. In theory, by means of a systematic scanning of a dense discrete set of possible central frequencies, it is feasible to evaluate the CFO from the detection of the PSS but such a systematic scanning would require a great amount of digital processing resources and thus increase the processing costs of the receiver.
It is therefore desirable to have an alternative technique which can achieve CFO estimation with limited digital processing resources.